Differential expression of major histocompatibility complex class I molecules in the brain of a New World monkey, the common marmoset (Callithrix jacchus)

It has been supposed that central nervous neurons do not express MHC class I molecules. However, recent studies clearly demonstrated functional MHC class I expression in the rodent brain. In the present study, we have extended these studies and investigated the presence of MHC class I transcripts and proteins in the brain of a non-human primate species, the common marmoset monkey (Callithrix jacchus). Using in-situ hybridization, we found strong expression of MHC class I transcripts in neocortex, hippocampal formation, substantia nigra and nucleus ruber. In-situ hybridization with emulsion autoradiography demonstrated MHC class I mRNA in distinct pyramidal neurons of cortex and hippocampus, in granule neurons of the dentate gyrus, in dopaminergic neurons of substantia nigra and in motor neurons of nucleus ruber. Immunocytochemistry confirmed MHC class I protein expression in these neurons. Two monoclonal antibodies, MRC-Ox18 and HB115, reacted differentially with MHC class I proteins on neuronal and non-neuronal cells, respectively. Interestingly, in marmoset monkeys that were immunosuppressed with FK506 (tacrolimus), expression of neuronal MHC class I proteins, which could be detected with MRC-Ox18, was either very low (neocortex, nucleus ruber, substantia nigra) or absent (hippocampus). In contrast, class I expression in endothelial cells, which was detected by HB115, was not affected by immunosuppression. Our data show that selected neurons in the brain of a non-human primate express MHC class I molecules and that this expression can be modulated by immunosuppression.